1. Field of Invention
The invention generally relates to optical amplifiers. The invention more particularly relates to a method and apparatus for controlling an optical amplifier exhibiting gain latency that is particularly well-suited to operating the amplifier at low pump output power and/or low gain values.
2. Description of Related Art
An Erbium-Doped Fiber Amplifier (EDFA) is a common and essential component in many fiber-optic communications networks. As the optical signal travels through the many kilometers of fiber optics, losses are incurred, and amplification of the signal becomes necessary.
The block diagram of an EDFA is quite simple as illustrated in FIG. 1. Essentially, a xe2x80x9cPumpxe2x80x9d light source (typically a pump laser 2 as shown) at either 980 nm or 1480 nm is summed in with the input optical signal 1 by using a coupler 4, and then the combined signal is sent through a length of erbium-doped fiber 6. The pump source 2 excites the erbium atoms in fiber 6 is such a way as to cause them to release multiple photons at the xcx9c1550 nm wavelength for every single input photon at xcx9c1550 nm. At the end of the erbium-doped fiber 6, the pump source 2 has been substantially absorbed, and the input 1550 nm signal 1 has been amplified to produce amplified output signal 8. Some portion of the energy from the pump source 2 has been transferred to the input signal 1 to achieve this amplification.
FIG. 1 shows a very simplified block diagram of an EDFA. Practical implementations may contain additional or substitute optical components and may be somewhat more complex. For example, instead of a coupler 4 a thin film filter or other conventional element may be utilized to combine the pump light with the input signal 1 light.
FIG. 2 illustrates other details of a conventional EDFA including a power supply 9 driving a variable current source 5 that feeds a DC drive current to pump laser 2. The conventional technique for controlling the gain and output power of the EDFA 7 is to control the variable current source 5. In other words, if more gain is desired then the current source 5 may be controlled to supply a higher DC current to the pump laser 2 which, in turn, causes pump laser 2 to output pump light having a higher optical power.
Most often, a variable current source 5 is used to bias the pump laser 2, as shown in FIG. 2. The pump laser 2 power output increases as the laser driving current is increased. Not shown in FIG. 2 is the usual closed-loop power control, where a conventional xe2x80x9cback-facetxe2x80x9d current is sensed by, for example, a back-facet PIN diode inside the laser package and used to control the current-source 5 in closed-loop fashion.
The complexity of fiber-optic networks is increasing, and amplifiers with very wide gain range are becoming useful. Conventional EDFAs such as those shown in FIGS. 1 and 2 that are designed for high gain applications, however, are not capable of producing low gain values that are stable and controllable. To produce a high gain EDFA a relatively long length of Erbium doped fiber 6 is pumped by a laser(s) utilizing a relatively high level of pump power. Likewise, low gains are possible by using a rather short length of Erbium doped fiber and a reasonably high pump power. The challenge is to implement an optical amplifier capable of high gain but still able to be throttled back to produce a stable low gain.
More specifically, an amplifier containing sufficient erbium-doped fiber 6, and a pump 2 capable of producing sufficient power for realizing high gains may have operational problems when required to operate at low gain or power levels. The pump power required at gains near unity may cause the laser 2 to be operated very near threshold. Some types of pump lasers may mode-hop more severely at these levels, and the back-facet power monitor may not produce an output signal sufficient to allow proper operation of the laser constant-power control circuitry.
FIG. 3 illustrates a source of the problems associated with conventional EDFAs operating at low output powers. Specifically, FIG. 3 plots output power of a conventional EDFA as a function of pump power (in this case a 980 nm pump laser is used). Below approximately 15 mW pump power, the corresponding EDFA output power enters a non-linear or otherwise highly unstable region. In other words, small variations in the pump power will cause large fluctuations in the EDFA output power. This is generally true of all conventional EDFAs.
FIG. 4 illustrates another difficulty of conventional EDFAs. Namely, the erbium absorption is a function of pump wavelength. FIG. 4 plots the normalized erbium absorption (a measure of how many of erbium ions have transitioned to higher energy states as a result of the pump light) as a function of pump wavelength. This wavelength dependency means that variations in the pump laser wavelength will vary the erbium absorption that, in turn, will vary the gain. This point is further illustrated in FIG. 5 which plots EDFA gain as a function of pump wavelength. Clearly, the EDFA gain may vary by 4 or 5 dB in this example depending upon pump wavelength. If the pump laser 2 is operated near its threshold (in this case approximately 10 mW) then mode competition may cause the pump wavelength to destabilize which, in turn, destabilizes the EDFA gain and output power.
FIG. 6 clearly exhibits this instability by plotting the pump power and back monitor current fluctuation (in percentage terms) as a function of pump light optical power. As can be seen, low pump powers cause large percentage fluctuations in the EDFA output power. Such pump power fluctuations are typically caused by pump mode competition. Significantly, such pump power fluctuations will transfer to EDFA gain and power fluctuations.
FIG. 7 shows even greater power fluctuations which are largely the result of a different type of pump laser 2. The pump laser associated with FIG. 6 is a Fabry-Perot laser while the pump laser associated with FIG. 7 is a Bragg-grating stabilized laser which is more susceptible to mode hopping or mode competition, particularly at low pump powers. Such mode hopping can destabilize or otherwise alter the pump output wavelength. As shown in FIG. 5, changes in the pump wavelength will cause gain fluctuations. Thus, severe mode hopping or competition at low pump powers is a serious problem that causes unwanted EDFA gain and output power fluctuations.
This invention includes a method and system for overcoming the difficulties of operating pump lasers at the low powers necessary for low EDFA gains. The result is to allow implementation of EDFA and other amplifiers with relatively wide controllable variable gain.
To that end, the inventive optical amplifier was developed that includes a gain media exhibiting gain latency and receiving an input optical signal; an optical pump optically coupled to said gain media, said optical pump supplying pumping light to said gain media, said pumping light having at least one wavelength sufficient to induce gain in the input optical signal; a current source operatively connected to said optical pump, said current source providing a drive current to said optical pump; a switch interposed between said current source and said optical pump; a modulator operatively connected to said switch; said modulator generating modulated signals having a duty cycle and supplying the modulated signals to said switch, said switch actuating in response to the modulated signal such that said switch repeatedly turns the drive current xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d.
The inventive optical amplifier may also include a controller operatively connected to said modulator, said controller supplying a control signal to said modulator to control the duty cycle of the modulated signal, said modulator generating modulated signals in response to the control signal.
The controller may also be operatively connected to said optical pump such that said controller receives a feedback signal from said optical pump indicative of the pumping light generated by said optical pump, said controller utilizing the feedback signal to control the duty cycle of the modulated signal.
The controller may also receive a target indicative of a target pump power level to be achieved by said optical pump and the controller may feedback control said modulator according to the feedback signal such that said optical pump substantially achieves the target. Alternatively, the controller may store the target rather than receive it.
Alternatively, the invention may also include an optical power monitor optically coupled to an output of the optical amplifier; said controller also operatively connected to said optical power monitor, said controller receiving a feedback signal from said optical power monitor indicative of an optical power of the signal output from the amplifier, and said controller utilizing the feedback signal to control the duty cycle of the modulated signal.
In another construction consistent with the invention a receiver may be optically coupled to an output of the optical amplifier; a power monitoring circuit operatively may be coupled to said receiver; and said controller may be operatively connected to said power monitoring circuit such that said controller receives a feedback signal from said power monitoring circuit indicative of an optical power of the signal output from the amplifier and utilizes the feedback signal to control the duty cycle of the modulated signal.
The modulator may be a pulse-width modulator generating pulse-width modulated signals in response to the control signal and supplying the pulse-width modulated signals to said switch. In this construction, the controller may supply the control signal to control a duty cycle of the pulse-width modulated signal.
The modulator may also be a frequency modulator that modulates a frequency of pulses having a fixed pulse-width, generates frequency modulated signals in response to the control signal, and supplies the frequency modulated signals to said switch. In this construction, the controller may supply the control signal to control a duty cycle of the frequency modulated signal.
The inventive gain media may take many forms such as a length of erbium doped fiber, a length of thulium doped fiber, an erbium doped waveguide, or a thulium doped waveguide.
Moreover, the optical pump may include a back-facet diode, the back-facet diode supplying the feedback signal to said controller.
The invention may also be constructed such that the current source and the switch are integrated in a switched current source operatively connected to said optical pump and to said modulator, wherein said switched current source actuates in response to the modulated signal such that said switch repeatedly turns the drive current xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d.
The invention may also be characterized as a power control circuit for an optical amplifier exhibiting gain latency and having an optical pump powered by a drive current from a current source, including: a switch interposed between the current source and the optical pump; and a modulator operatively connected to said switch; said modulator generating modulated signals having a duty cycle and supplying the modulated signals to said switch, said switch actuating in response to the modulated signal such that said switch repeatedly turns the drive current xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d. All of the above configurations, constructions and functionalities described above also apply to the inventive power supply circuit.
In addition, the invention includes a method of controlling an optical amplifier exhibiting gain latency and having an optical pump driven by an electrical current, comprising: switching the electrical current supplied to the optical pump xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d; and controlling said switching with a modulated signal having a duty cycle in order to control a relative duration of the xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d periods of the electrical current supplied to the optical pump, wherein the gain latency in the optical amplifier permits a substantially continuous signal to be output from the optical amplifier in response to the switched electrical current supply to the optical pump.
The inventive methods further include receiving a feedback signal from the optical pump indicative of the pumping light generated by the optical pump, and utilizing the feedback signal to control the duty cycle of the modulated signal.
The inventive methods may also include inputting a target indicative of a target pump power level to be achieved by the optical pump; and feedback controlling the duty cycle of the modulated signal according to the feedback signal such that the optical pump substantially achieves the target.
The inventive method may further include receiving a feedback signal from a power monitoring device indicative of an optical power of the signal output from the amplifier, and utilizing the feedback signal to control the duty cycle of the modulated signal.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.